Is it possible to write a C program without using header files?

Question

Is it possible to write a C program without using header files? If it is, how?

Answer 1

Of course:

int main() {
   return 0;
}

Or even:

int printf(const char *format, ... ); // could be copied from stdio.h

int main() {
   printf("Hello, world!\n");
   return 0;
}

The #include directive effectively just includes the header file's content in the source file.

Answer 2

Of course.

A header file is just a file that gets included in some source files, and when you include a file you just copy its content.

You can write any program you want to without any #include, but you'd have to manually put the stuff you need in your source files.